An interconnect circuit board of an LTCC design is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. Use of a Low Temperature Cofired Ceramic (LTCC) multilayer structure allows a circuit to be more compact than traditional alumina substrates by allowing vertical integration.
LTCC tape has been used in prior art high frequency microwave applications, such as telecommunications, automotive or military applications including radar, for its multilayer, cofiring and flexible design capabilities. Many properties are required of the conductors used in the fabrication of multilayer circuits for microwave applications, including desired resistivity, solderability, solder leach resistance, wire bondability, adhesion, migration resistance, and long term stability. Additionally, manufacturers are now seeking thick film conductor compositions for use in high frequency microwave applications wherein the compositions minimize lead and cadmium content in the circuits to meet environmental concerns.
U.S. Pat. No. 5,744,232 to Bailey discloses a thick film metallization compatible with LTCC that display very low microwave insertion losses commensurate with those of thin film gold. The electrical performance of the metallization is attained by using a spherical metal particle shape and uniform particle size distribution in the thick film paste. Bailey does not disclose the use of a lead free, cadmium free thick film conductor.
Furthermore, some prior art thick film conductor compositions used in high frequency applications have been known to require a “double print” (i.e., more than one application of the conductor composition on the substrate) to meet the desired properties of the system, in particular, to meet satisfactory wire bond acceptance and adhesion properties.
It is especially difficult to maintain the desired thick film conductor properties while minimizing or eliminating lead and cadmium. The present inventors provided such a lead free and cadmium free conductor that simultaneously maintains the desired thick film conductor properties and requires only one application of the thick film on the substrate to meet wire bonding requirements.